The present invention relates generally to refractometers for measuring the refractive index of a translucent material during a physical or chemical process. More particularly, the invention relates to refractometers for measuring the refractive index of a transparent polymeric material during a curing process. Even more particularly, the invention relates to refractometers for continuously measuring the dynamic refractive index of a photocurable resin or adhesive mixture during the process of curing the mixture by means photonic irradiation.
Growing demand for photocurable polymeric materials has necessitated the development of advanced process monitoring techniques to ensure high quality production of cured polymers at minimum cost. In particular, monitoring the progress of the cure cycle of a photocured polymeric article during the polymerization process is critical to ensuring complete polymerization of the polymer matrix of the processed article while also providing for the shortest unit production times and the greatest production throughput of such photocured articles.
The degree of polymerization of the polymer matrix of a processed article is directly affected by various physical processing parameters. Conventionally, these parameters are varied by preset amounts over the entire curing cycle based on a predetermined cure cycle. However, even if an optimal cure cycle is determined for a given chemical composition and application, uncontrolled or unobserved variations in the temperature, pressure, and/or intensity and duration of photonic irradiation used during cure still result in variations in the degree of polymerization of the polymer matrix. It is therefore essential to reduce the effect of processing variations by active control of the cure cycle rather than simply using predetermined values for application of the processing parameters. Such a scheme requires dynamic properties of the curing polymeric mixture to be measured throughout the cure process. These measured dynamic properties would then serve as an input to a process model, which in turn would be used in situ to control the processing parameters during processing. Such a processing scheme would produce polymeric articles of consistent quality at maximum speed while reducing waste due to improper curing.
In particular, a device to continuously and accurately measure the cure process of photocurable polymers would provide an input to the process model so as to allow optimal control of selected processing parameters such as total cure time and photonic irradiation duration and intensity. One measurable dynamic parameter that accurately indicates the status of the curing process in transparent or translucent photocurable polymers is the refractive index of the polymeric material. Generally, the refractive index of a photocurable polymeric material changes predictably and continuously from the refractive index of the material in an uncured state to the refractive index of the material in a completely cured state.
Thus, there is a need for simple and fast method and device to continuously and accurately measure the changes of refractive index of a photocurable polymeric mixture during transformation from uncured raw materials into a cured polymer. Such a device and method would be of great value since photocurable polymers, such as photocurable resins and adhesives, are frequently used in a wide variety of optical applications such as assembling optical components, making compound lenses, creating and connecting optical fibers, fixing fibers in mechanical splices, and other similar applications.
Refractometers are one class of devices commonly used to measure refractive index of materials. In general, refractometers measure the critical angle of total reflection by directing an obliquely incident beam of light at a surface-to-surface boundary between a high refractive index prism and a translucent sample to allow a portion of the light to be observed after interaction at the boundary. Reflected light refractometers detect and measure the light that is reflected at the surface-to-surface boundary. In prior art reflected light refractometers, an illuminated region is produced over a portion of a detection field of view, and the location of a shadowline between the illuminated region and an adjacent dark region in the detection field of view allows the sample refractive index to be deduced geometrically.
Current devices used to measure the changes of refractive index during the cure cycle of photocurable polymers include classical ABBE Type refractometers. ABBE Type refractometers measure either the refractive index of the liquid polymeric resin prior to initiation of the cure process or measure the refractive index of the cured polymer after the completion of the curing process. A significant time delay between the change of the refractive index and the moment when the data is obtained makes real time or even near-real time measurement of the kinetics of the refractive index change during the photo-curing process impossible.
Prior art refractometers include a prism coupler described in paper R. Ulrich and R. Forge. “Measurement of thin film parameters with a prism couple”, Appl. Optics 12 (1973) 2901-2908 and a prism coupler found in the Metricon 2010 made by Metricon Corporation, P.O. Box 63 Pennington, N.J. 08534, telephone 609-737-1052. The principle of operation used in these two prior art devices is based on the measurement of the critical angle of total internal reflection. A material with unknown refractive index is put in contact with the long side face of a rectangular isosceles prism. The prism must have refractive index greater than of the material. A laser beam enters the prism from one of its short side faces. The beam strikes the interface between the resin and the prism and is reflected due to the total internal reflection towards the second short side face of the prism. After exiting the prism the beam strikes a silicon photodetector, which generates signal proportional to the intensity of the beam. The prism, together with the resin in contact with it, are mounted on a rotary table which rotates with respect to the fixed laser beam until the intensity of the totally internally reflected portion of the beam sharply drops down. The angle of incidence of the beam striking the interface at this position of the prism is equal to the critical angle of reflection. The unknown refractive index of the material could be calculated using the known index of the coupling prism and the critical angle.
One disadvantage of these prior art measurement instruments is that the measurement of the refractive index using these devices is inherently static. Measurement of any change of the refractive index must be based on the change in the critical angle. Changes in the critical angle require a separate series of rotations of the prism for each discrete measurement and also require, for each discrete measurement, determination of the change in the angular displacement at which the intensity of totally internally reflected beam abruptly changes as compared to the previous discrete measurement. Such, rotation, either manual or automatic, is a time consuming procedure that is not suitable for continuous tracking of the changing critical angle. A second disadvantage is that the prior art devices do not incorporate a source of photonic radiation with the mechanism for measuring refractive index of the photocurable polymeric materials. Combining both functions into one device would provide a highly desirable economy in the cost of equipment.
What is needed, then, is a refractometer which provides continuous, real-time measurements of the dynamic changes in the refractive index of a translucent photocurable material during the curing process.
Additionally, what is needed is a refractometer which provides continuous, real-time measurements of the dynamic changes in the refractive index of a translucent photocurable material during the curing process and which provides a source of photonic irradiation for curing the material.
Yet additionally, what is needed, is a system or method for controlling the processing parameters during the process of curing a translucent photocurable material wherein the system or method incorporates a refractometer to provide continuous, real-time measurements of the dynamic changes in the refractive index of the translucent photocurable material and further provides an apparatus or steps to adjust the processing parameters of the curing process based on such measurement of real-time changes in refractive index.